Dude I really do not think they were hitting with the kind of power you are talking about man. Seriously. Besides what makes you think the portal has multiversal or even universal-level power? That's silly. Dude it's just a fictional wormhole-generator. Just because it can GO to countless different universes doesn't mean that the portal-generator itself somehow has the energy of many universes! That's a HUGE stretch of an assumption to make, man.
Bill was not using his full power fighting the Shacktron dude. I mean its hand was still only a T-Rex head. It's nto like it coudl somehow siphon its sci-fi energies into the T-Rex. The T-Rex pulled out Bill's eye. Lol. The whole thign was just silly.
If it was hitting with the kind of force you are talking about, why did we not see corresponding damage to the surrounding environment? The planet should have been destroyed, realistically, but at the very least they would have depicted HUGE damage.
Finally, the portal may somehow provide power for the device but it isn't fully operational and connected to the multiverse there in the Shacktron. Look at its appearance. It has not fully opened upon the multiverse, you can tell because
it is a solid blue. When the portal fully activates, it takes on the appearance of concentric or spiraled (forget which, but it was one of the two) MULTICOLORED RINGS.
The portal, fully opened and actually connected to the multiverse, risks destroying the entire universe every time it is opened. No matter what, McGucket AFTER REGAINING HIS SANITY and after being the one in the past to say the portal should be dismantled, would never have dared to use the fully-opened portal as part of a WEAPON, seriously.
Sure it's NICE to think that the mech was as powerful as you are saying, but overall I just think it was clearly not depicted that way.
Oh, also, if the portal was activated and being used as a power-source for the weapon, why weren't there any gravitational or other anomalies? And why was it never MENTIONED? You'd think something that significant would be brought-up out loud in the show, like "oh yeah so this super-dangerous portal that can potentially destroy the universe? Well sicne Bill is even more dangerous we're gonna use it to power this weapon." They would have brought that up.
Overall I think that possibly the portal is only there as part of the Shacktron to make it look cooler

Haha.